The ligand binding domain of the orphan nuclear receptors TR4 and TR2
The ligand binding domain of the TR4 and TR2 (human testicular receptor 4 and 2): TR4 and TR2 are orphan nuclear receptors. Several isoforms of TR4 and TR2 have been isolated in various tissues. TR2 is abundantly expressed in the androgen-sensitive prostate. TR4 transcripts are expressed in many tissues, including central nervous system, adrenal gland, spleen, thyroid gland, and prostate. The expression of TR2 is negatively regulated by androgen, retinoids, and radiation. The expression of both mouse TR2 and TR4 is up-regulated by neurocytokine ciliary neurotrophic factor (CNTF) in mouse. It has shown that human TR2 binds to a wide spectrum of natural hormone response elements (HREs) with distinct affinities suggesting that TR2 may cross-talk with other gene expression regulation systems. The genes responding to TR2 or TR4 include genes that are regulated by retinoic acid receptor, vitamin D receptor, peroxisome proliferator-activated receptor. TR4/2 binds to HREs as a dimer. Like other members of the nuclea r receptor (NR) superfamily of ligand-activated transcription factors, TR2-like receptors have a central well conserved DNA binding domain (DBD), a variable N-terminal domain, a flexible hinge and a C-terminal ligand binding domain (LBD).